The invention relates to a voltage-to-current converter, comprising an input for receiving an input signal, an output for supplying an output signal, a first transistor of a first conductivity type, having its emitter coupled to a reference-voltage point via an impedance and having its collector coupled to the output via the base-emitter junction of a diode-connected second transistor, the emitter-base junction of a third transistor of a second conductivity type being arranged in parallel with the base-emitter junction of the second transistor, the collector of said third transistor being coupled to the base of the first transistor and also being coupled to the input via the emitter-base junction of a fourth transistor of the second conductivity type which has its collector coupled to the emitter of the first transistor.
Such a voltage-to-current converter is known from the international Patent Application No. WO 86/04196 and, in principle operates in the same way as the well-known emitter-follower arrangement if the current in the collector instead of the voltage on the emitter of the first transistor is taken as the output signal.
An emitter-follower arrangement has the drawback that the output current increases as a non-linear function of the input voltage, because the base-emitter voltage of the first transistor varies as the input voltage increases.
In the known-voltage-to-current converter the influence of the base-emitter voltage of the first transistor on the output signal is compensated for by coupling the base of the first transistor indirectly to the input via the base emitter junction of the fourth transistor, which is of a conductivity type opposite to that of the first transistor. Compensation is achieved by equalizing the base-emitter voltage of the first and the fourth transistor by means of the second and the third transistor which are arranged as a current mirror and which are of the same conductivity type as the fourth transistor. The current mirror maintains tha collector current of the first transistor equal to the emitter current of the fourth transistor, so that the base-emitter voltages of the first and the fourth transistor are approximately equal.
The compensation accuracy depends on the mirror ratio of the current mirror and the ratio between the saturation currents of the first and the fourth transistor. Since the first and the fourth transistor are of opposite conductivity types fixing the ratio between their saturation currents is comparatively problematic in practice. In practice the mirror ratio of the current mirror will also exihibit an inaccuracy owning to the spread in transistor parameters. As a result of this, the compensation of the base-emitter voltages of the first and the fourth transistor in the known voltage-to-current converter will be incomplete, which adversely offers the accuracy of the voltage-to-current conversion. It is an object of the invention to mitigate the above drawbacks of the known voltage-to-current converter and to improve the accuracy of the voltage-to-current conversion.